When Donald Kessler's head wakes up inside the alien space-craft, we can hear a steady heartbeat. However, when the camera pans out, and we are able to see the heart to the left of the screen, the heart is beating much faster than the speed the audio is beating.
Richie brings home donuts to the trailer. His mother asks how old they are, to which he says, "Monday," and she retorts, "that's six days ago," making it Sunday - yet the opening scene says it is Thursday morning.
When the general says that they have to attack with nuclear weapons, and shouts; "It already is war!" and the president responds; "Shut up, shut up, SHUT UP!" you can see the general's clipboard change place from under the general's arm and to the desk and back again.
When General Casey arrives at the Landing Site in Pahrump, there are only about 100-200 civilians and a few reporters present, but in the shot of Barbara overlooking the Landing Site on the hill, the site is filled with several thousand people.
When the Martians land in the desert for the official greeting, the TV in the White House is tuned to the commentary being broadcast by Jason Stone, but a minute later it is showing the report from Nathalie Lake, who works for a different channel.
The "Martian Girl" is standing in front of the Washington Monument when the White House Press Secretary rolls up in his limousine and offers to show her around the White House, which we see just across the street. Actually, the Washington Monument and White House are about 1/2 mile apart.
When the Houses of Parliament are shown during the Martian attack, they are shown in reverse. The tower with Big Ben is to the left of the screen, when in fact it would be to the right of the screen as the shot is from the south bank of the river Thames.
In one of the Kansas scenes, the camera pans across the farm and surrounding areas. The faint skyline of the University of Kansas (in Lawrence) can be seen, though the scene apparently takes place elsewhere.
The bus driver Louise Williams sees her kids Cedric & Neville in a games arcade and says to her passengers:
"Excuse me, folks but we've got to make an unscheduled stop!"
It takes about four seconds between braking and stopping. At 30 mph that would have taken about 120 feet yet she manages to stop the bus at the arcade about 10 feet after she first saw the kids.
Taffy Dale presents Florence Norris and Richie Norris with the Congressional Medal of Honour and the Medal of Honour respectively. The Medal of Honour is exclusively a military award. There are three versions of the medal, one for the Army, one for the Navy, and one for the Air Force. Personnel of the Marine Corps and Coast Guard receive the Navy version. The Congressional Gold Medal and the Presidential Medal of Freedom are the highest civilian awards in the United States. Florence Norris and Richie Norris were civilians and should have received the civilian medals.
When General Decker is preparing his pistols in the war room, he pulls back the slide, then pushes the slide release to release it forward - this is only necessary to do if the weapon is not loaded; on a loaded gun, racking the slide chambers a round and lets the action move into the forward position, ready to fire.
When Cedric and Neville Williams leave their video game to talk to their dad on the phone, the video game they were playing continues to rack up points. Also, the first time we see them in the movie, they are at an arcade paying quarters to play the very same game.
The goof items below may give away important plot points.
At the end of the movie Granny and Richie are awarded Congressional Medals of Honor. The CMH is only given to members of the armed forces - they should have been awarded the Presidential Medal of Freedom.
When the Galaxy is destroyed, an African-American woman on the street wearing a gold dress drops her purse and begins running when the Martian ship fires, but in the shot of the people fleeing on the street, her purse is back with her, with no time for her to have gone back to get it.
The Martians breathe nitrogen and need to have it supplied through their life vests or, in the case of the Martian girl infiltrator, through special gum. However when the scientists analyze the special gum they say that it is "nitrogen, NO2". NO2 is called nitrogen dioxide and is poisonous to humans, while nitrogen is the primary component of the Earth's atmosphere which means that the Martians probably didn't need any extra.
When the Martians infiltrate Tom Jones' concert, the skeletons of his three backup singers are seen, but the Martians' ray-guns were never heard firing, (nor did any of the audience or band members apparently see the singers' flesh burn up, the flash of the ray-guns, or anything else out of the ordinary) and the music was not loud enough to drown out the sound.